The present invention relates to a hybrid type stepping motor (hereafter referred to as a motor) used for OA equipment treating images, such as a facsimile, an ink-jet printer, a laser beam printer, or a copy machine.
A motor used for each above OA equipment is required to reduce vibration and noise with satisfying cost reduction as the absolute condition.
U.S. Pat. No. 6,781,260 discloses a stepping motor that has large torque with low vibration. The stepping motor disclosed in the publication is constructed by arranging a rotor having two sets of rotor units inside a stator on which windings are wound. Each of the rotor units consists of a ring-shaped unipolar magnet whose flat surfaces are magnetized and a pair of rotor cores that sandwich the magnet. Many rotor teeth are formed around each of the rotor cores. The two rotor units are attached to a motor shaft so that the magnets have opposite polarities to make the magnetic polarities of the rotor teeth of the adjacent two rotor cores identical. Since the stepping motor of the publication is provided with four rotor cores, radial attraction forces are distributed and balanced as compared with a conventional motor with two rotor cores. Therefore, an imbalance moment does not occur, which reduces vibration and noise owing to clearances of bearings or the like. That is, vibration and noise are lower than the conventional motor. This stepping motor generates double the torque of the conventional motor of the same physique theoretically.
Generally, in such a stepping motor, it is necessary to serialize products so that a user can select motor torque according to load conditions used. In many cases, motors that are identical in the radial size but are different in length in the axial direction are prepared in order to serialize motors with different output torques. It is known for the conventional common motor that the optimum magnetic flux density distribution exists in the magnetic circuit that is determined by a residual magnetic flux density Br of a magnet, and configurations of a rotor core and a stator core.
However, since the disclosed stepping motor in the U.S. Pat. No. 6,781,260 is the first of its type in which two sets of rotor units each of which has a ring-shaped unipolar magnet whose flat surfaces are magnetized are attached to the motor shaft so that the magnets have opposite polarities, the optimum magnetic flux density distribution has not been analyzed.
In the meantime, the Japanese unexamined utility-model publication No. 52-59212(1977-59212) discloses a stepping motor with 8-pole stator structure that uses a plurality of ferrite magnets with a small residual magnetic flux density.
However, this publication does not describe a magnetic circuit and it does not suggest how to obtain the optimum magnetic circuit.
On the other hand, most stepping motors are designed that an air gap between an outer circumference of the small rotor teeth and a tip of a stator main magnetic pole is 0.05 mm in order to keep their performance. Therefore, extremely high precision is required for the roundness of rotor and the coaxiality of a rotor outer surface with respect to the motor shaft. In the past, a rotor is formed by stacking and unifying stamped core plates and then polishing an outer surface of the small rotor teeth in order to keep the required roundness and coaxiality.
Similarly, since extremely high precision is required for the roundness of the inside surface of a stator, a finishing process called honing is applied. Such a process keeps the air gap of about 0.05 mm between the outer surface of the small rotor teeth and the inner surface of the stator.
If a polishing process of the outer surface of the small rotor teeth causes a burr, it may be removed and get into the air gap, which may lock the motor. Therefore, a process to prevent a burr on the outer surface of the rotor is executed before the polishing process of the outer surface of the small rotor teeth. For example, resin is applied to coat around the outer surface of the rotor to avoid dispersion of a burr. In such a case, the outer surface is polished after the resin cures. Alternatively, a shot blast process, a buff polishing, or a process using water pressure may be applied after polishing the outer surface of the rotor.
Thus, there are many factors to increase the cost in order to keep the air gap of about 0.05 mm between the outer surface of the small rotor teeth and the inner surface of the stator. The factors include the polishing process for the outer surface of the rotor, the honing process for the inner surface of the stator, the anti-burr surface treatment, etc.
So, Japanese unexamined patent publication No. 2005-6375(hereinafter, called JP2005-6375A) discloses a manufacturing method of a rotor by forming a core plate that has many small rotor teeth around its periphery through stamping with a punch press and stacking a plurality of core plates. In this method, the press process is divided into a first step to form small holes by following the inner surfaces of the small rotor teeth and a second step to stamp the plate by following the outer surfaces of the small rotor teeth. As a result of the two-step process, the edge surface of the small tooth does not deform, and thereby the polishing process for the outer surface of the rotor becomes unnecessary.
Although the technique of JP2005-6375A can omit the polishing process, it requires two-step press process, which also increases the manufacturing cost.
In a standard stepping motor with an air gap of 0.05 mm, an attraction force in a radial direction between a rotor and a stator becomes extremely large against a generating torque, and such an attraction force becomes a source of an exciting force of the motor.
Japanese unexamined patent publication No. 2000-197335(hereinafter, called JP2000-197335A) suggests a specific arrangement of all pillars of magnetic pole and a specific arrangement of small rotor teeth in a circumferential direction as an improvement in vibration and noise of a motor with a rectangular stator.
However, the method of JP2000-197335A is not practical because shape and size of an applicable motor are limited.
Further, the stepping motor disclosed in the above-mentioned U.S. Pat. No. 6,781,260 discloses that an air gap between the outer surface of the small teeth of the rotor core and the inner surface of the tips of the stator main magnetic poles is about 0.05 mm in general. Therefore, the stepping motor of U.S. Pat. No. 6,781,260 also requires the surface treatment to keep high accuracy in roundness of a rotor core and a stator.